1. INTRODUCTION

Thirty years ago, cosmology was described as a search for two numbers:
the current expansion rate (or Hubble constant), H0,
and its change
over time, the deceleration parameter, q0
(Sandage,
1970).
But that was before the discovery of giant walls of galaxies, voids, dark
matter, tiny variations in the cosmic microwave background radiation
(CMB), dark
energy and the acceleration of the Universe. Today, the subject has
become vastly richer, and the numbers being sought are more numerous
but more closely tied to fundamental theory. An overall picture has
emerged that accounts for the origin of structure and geometry of the
Universe, as well as describing its evolution from a fraction of a
second onward.

In this new and still-evolving picture rooted in elementary particle
physics, in a tiny fraction of a second during the early history of
the universe, there was an enormous expansion called inflation. This
expansion smoothed out wrinkles and curvature in the fabric of
spacetime, and stretched quantum fluctuations on subatomic scales to
astrophysical scales. Following inflation was a phase when the
Universe was a hot thermal mixture of elementary particles, out of
which arose all the forms of matter that exist today. Some 10,000
years into its evolution, gravity began to grow the tiny lumpiness in
the matter distribution arising from quantum fluctuations into the
rich cosmic structures seen today, from individual galaxies to the
great clusters of galaxies and superclusters.

Recent observations of the universe have not only strengthened and expanded
the big-bang model, but they have also revealed surprises. In
particular, most of the universe is made of something fundamentally
different from the ordinary matter we are made of. (By ordinary
matter, we mean matter made of neutrons and protons; the jargon for
this is baryons, the technical term for particles made of quark
triplets.) About 30% of the
total mass-energy is dark matter, composed of particles most likely
formed early in the universe. Two thirds is in a
smooth "dark energy" whose gravitational effects
began causing the expansion of the universe to speed up just
a few billion years ago. Ordinary matter, the bulk of it dark, only
accounts for the remaining 4% of the total mass-energy density of
the universe.
While the remnant (thermal) microwave background from the hot big bang
contributes only about 0.01%, it encodes information about the
spacetime structure of the Universe, about its early history,
and possibly even about its ultimate fate.

We have also learned much about the organization of the
universe. In the nearby universe, galaxies are distributed in a
"cosmic web" composed of sheets and sinuous filaments interspersed
with voids (see Figures 1 and
2). Though inhomogeneous on these apparently
vast scales, the Universe becomes more and more homogeneous when
viewed on even larger scales from 100 Mpc out to the current horizon
of 10,000 Mpc.

Figure 1. The Universe observed:
A slice of the Universe constructed from the positions of
60,000 galaxies in the Sloan Digital Sky Survey. Voids and walls can
be clearly seen (image courtesy of SDSS).

Figure 2. The Universe simulated:
The distribution of dark matter in a large-scale
numerical simulation of the Universe. The cosmic web of dark matter -
with its sheets, sinuous filaments and voids is apparent (image courtesy of
the Virgo Consortium).

In the first part of this review, we describe the universe - its
structure, composition and global properties. Then we proceed to
discuss our current understanding of its origin and early evolution,
emphasizing the deep connections between physics on the smallest and
largest scales. We end by discussing some recent and more speculative
ideas from theory, as well as posing some of the "big questions"
confronting cosmology today.